The present invention relates to a temperature detecting circuit, especially to a temperature detecting circuit that performs temperature detection by utilizing temperature-voltage characteristics of circuit elements in semiconductor integrated circuits, and to a liquid crystal driving device that compensates temperature characteristics of a liquid crystal element with a driving voltage in accordance with the detection result.
Disclosed in Japanese Unexamined Patent Publication Tokukaihei No. 3-48737 (published on Mar. 1, 1991) is typical conventional technology as the above-mentioned circuit for the temperature detection by utilizing the temperature-voltage characteristics of circuit elements in semiconductor integrated circuits. FIG. 7 is a block diagram showing an electric configuration of a temperature detecting circuit of the conventional technology. This conventional technology is provided with a first bias voltage source b1, a second bias voltage source b2, and an amplifier 3. The first bias voltage source b1 is configured by connecting a series circuit, which includes a constant current source f1 and a plurality of diodes d11 to d1n, between power supplying lines 1 and 2, while the second bias voltage source b2 is configured by connecting a series circuit, which has a constant current source f2 and a plurality of diodes d21 to d2m, between the power supplying lines 1 and 2. The amplifier 3 is for amplifying and outputting a difference between first and second bias voltages from the first and the second bias voltage sources b1 and b2, respectively. A junction between the constant current source f1 and the diode d1n is an output terminal for the first bias voltage, and is connected to one of two input terminals of the amplifier 3, while a junction between the constant current source f2 and the diode d2m is an output terminal for the second bias voltage, and is connected to the other input terminal of the amplifier 3.
Because nxe2x89xa0m, when current values of the constant current sources f1 and f2 are equal to each other, a voltage of xe2x88x92nxc3x97Vac [V] is generated at one of the input terminals of the amplifier 3, while a voltage of xe2x88x92mxc3x97Vac [V] is produced at the other input terminal, where a voltage between anode and cathode of a single diode is Vac [V] and a potential of the power supplying line 1 is the reference. As a result, an offset of (mxe2x88x92n)xc3x97Vac [V] is generated between the two input terminals. Therefore, where the temperature dependence of the voltage between anode and cathode of a single diode is xcex94Vac [V/xc2x0 C.], a change in temperature by T [xc2x0 C.] varies the offset between the input terminals of the amplifier 3 by Txc3x97(mxe2x88x92n)xc3x97xcex94Vac [V]. Thus, Axc3x97T (mxe2x88x92n)xc3x97xcex94Vac [V] is obtained when A is the gain of the amplifier 3.
In the above-mentioned conventional technology, because the differences between two voltages, namely one from the diodes d11 to d1n of the first bias voltage source b1 and the other from the diodes d21 to d2m of the second bias voltage source b2, are outputted as the detected temperature, the temperature detection can be carried out with relative accuracy between the first and the second bias voltage sources b1 and b2, as long as element characteristics of the respective diodes, namely d11 to d1n and d21 to d2m are equal. Thus, the temperature detection can be performed with high accuracy without requiring individual elements to be highly accurate.
The problems of the technology are that sensitivity of the temperature detection is not arbitrarily adjustable and the output voltage cannot be amplified to a desirable level. Especially, a liquid crystal panel has some characteristics changed significantly depending on ambient temperature, such as relationship of applied voltage-light transmittance characteristics and threshold voltage Vth characteristics of the liquid crystal materials. Therefore, its driving voltage is required to be altered in accordance with the ambient temperature for displaying constantly with a most suitable contrast. Moreover, different types of materials of a liquid crystal element, or even an identical material with different thickness of liquid crystal layers will show some differences in the characteristics such as the threshold voltage Vth.
The object of the present invention is to provide a temperature detecting circuit that can adapt to various temperature characteristics and output dynamic ranges.
A temperature detecting circuit of the present invention includes an inverting amplifier for outputting a voltage in accordance with a difference between a first bias voltage from a first bias voltage source with relatively steep temperature characteristics and a second bias voltage from a second bias voltage source with relatively gradual temperature characteristics, the inverting amplifier outputting the voltage in accordance with a difference between the first bias voltage and the second bias voltage so as to perform temperature detection with relative accuracy between the first and the second bias voltage sources, the temperature detecting circuit comprising a first resistance for supplying the first bias voltage to an inverting input terminal of the inverting amplifier, a second resistance which is disposed between the inverting input terminal and an output terminal of the inverting amplifier, a non-inverting amplifier having a non-inverting input terminal for receiving the output from the inverting amplifier, a third resistance for supplying a predetermined reference potential to an inverting input terminal of the non-inverting amplifier, and a fourth resistance which is disposed between the inverting input terminal and an output terminal of the non-inverting amplifier.
In the above arrangement, the first bias voltage Vin from the first bias voltage source with the relatively steep temperature characteristics is supplied to the inverting input terminal of the inverting amplifier, while the second bias voltage Vbias from the second bias voltage source with the relatively gradual temperature characteristics is forwarded to the non-inverting input terminal of the inverting amplifier, and by disposing the first resistance R1 between the first bias voltage source and the inverting input terminal and the second resistance R2 between the inverting input terminal and the output terminal, the output voltage Vout1 from the inverting amplifier can be described as follows:
Vout1=xe2x88x92(Vinxe2x88x92Vbias)xc3x97R2/R1+Vbias.
Thus, the difference between the second and the first bias voltages, namely Vbias and Vin, is added to the Vbias, which is the second bias voltage with the relatively gradual temperature gradient, after multiplied by the ratio of the second resistance to the first resistance. Therefore, the temperature detection can be performed with the relative accuracy between the first and the second bias voltage sources. Moreover, desired temperature characteristics can be obtained by appropriately setting the resistivities of the first and the second resistances.
Furthermore, the output voltage Vout1 from the inverting amplifier is amplified by supplying it to the non-inverting input terminal of the non-inverting amplifier, which receives a fed-back output via the fourth resistance and the reference potential via the third resistance at the inverting input terminal.
Therefore, the temperature characteristics obtained by the inverting amplifier can have the desired output voltage value by appropriately setting the resistivities of the third and the fourth resistances.
Moreover, the temperature detecting circuit of the present invention includes the first and the second bias voltage sources, wherein the first and the second bias voltage sources respectively have series circuits connecting a constant current source and one or more stages of a diode or diodes, between power supplying lines, and supply the bias voltages to input terminals of the inverting amplifier from their respective junctions between the constant current sources and the one or more stages of a diode or diodes, so as to create the difference between the temperature characteristics by a difference in element area between the diodes of the respective bias voltage sources.
In the above arrangement, diodes having different current abilities, which are prepared to have different areas per diode between the first and the second bias voltage sources, or to have different numbers of parallel connections of diodes having the same area between the first and the second bias voltage sources, are operated by fixing their operating points by constant currents from constant current sources, thus having different temperature characteristics and easily packaging the diodes in a single semiconductor integrated circuit.
Furthermore, a liquid crystal driving device of the present invention comprises the temperature detecting circuit and utilizing the output voltage from the non-inverting amplifier for driving a liquid crystal element, the liquid crystal driving device having a gain of the inverting amplifier, which is determined by the first and the second resistances and adapts to temperature characteristics of a liquid crystal panel, and having an output voltage level, which is determined by the third and the fourth resistances and the reference potential and adapts to a voltage required for driving the liquid crystal element.
In the above arrangement, the gain of the inverting amplifier is adapted to the temperature characteristics of the liquid crystal panel, such as the relationship of applied voltage-light transmittance characteristics or the threshold voltage Vth, which are varied depending on the types of materials of the liquid crystal element or the thickness of the liquid crystal layers, by setting the resistivities of the first and the second resistances, while the output voltage level is adapted to the voltage necessary to drive the liquid crystal element by setting the third and the fourth resistances and the reference potential.
Therefore, by setting the first to the fourth resistances and the reference potential, an arbitrary driving voltage can be obtained with any temperature characteristics suitable for the liquid crystal panel in use, thus performing display constantly with the optimum contrast.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.